A collaborative international study was done to determine whether the current WHO FIX Complex Standard, 84/681 (contains significant amounts of Factors II, VII, and K in addition to Factor IX) is an appropriate standard for determining the potency of the more highly purified Coagulation Factor IX (Human) products. Samples assayed included three Coagulation Factor IX (human) concentrates manufactured by Baxter Healthcare Corporation, Alpha Therapeutics Corporation, and Armour Pharmaceutical Company and one Factor IX complex (contains Factors II, VII, and X in addition to Factor IX) manufactured by Baxter Healthcare Corporation. All participants used the one-stage assay for Factor IX. Samples were prediluted plasma to investigate the effects of protein concentration and composition on the potency determination. Lower potency values were obtained when samples were prediluted in buffer only then when prediluted in 1% albumin or FIX deficient plasma. The current WHO Standard for FIX Complex appears to be a suitable standard for potency determination of the more highly purified FIX concentrates.